A laser printer typically prints on a radiation- or thermally-sensitive recording medium in which some physical or chemical property of the medium is changed in response to receiving an amount of radiation or heat that exceeds some threshold level. Thus, the creation of the image is dependent on the intensity of the incident or writing radiation. For example, in a typical recording medium, there is a 1:3 intensity ratio from threshold excitation of the recording medium to its destruction. As a result, the writing radiation is required to be focused onto the recording medium as a spot which has a substantially smooth, uniform intensity thereacross where a substantially smooth, uniform intensity means that the peak to valley intensity ratio in the spot is to engineering tolerance smaller than the threshold to burn-off ratio of the recording medium.
In addition, laser printers may use laser diodes to provide writing radiation. But, their usual output, which is in the range between 20 to 50 mW, would require the use of more than one arranged in an array where the media needed more energy for exposure than was available with only one diode. As a consequence, a laser printer could utilize an array comprised of a multiplicity of such laser diodes to provide an amount of radiative power suitable for printing. For example, a typical laser printer application might utilize an array comprised of as many as ten such laser diodes fabricated on the same semiconductor chip.
However, such arrangements may have still further problems. In particular, if the laser diodes in the array are spaced too far apart from each other, the array and the accompanying electronics may be expensive and it also becomes more difficult to produce a smooth, uniform intensity spot. Conversely, if the laser diodes in the array are spaced too closely together, mode hopping and other physical effects occur resulting in non-uniform radiation output from the array. Nevertheless, it is possible to achieve a configuration with an "intermediate" spacing that solves the above-described problems. For example, an array where each laser diode on the semiconductor chip has a 6 micron by 1 micron active stripe overcomes the above-described problems when the diodes are spaced approximately 4 microns from each other. While such a spacing appears to solve the above-described, it causes the phase of the output field from the laser diodes to alternate by an amount substantially equal to 180.degree.. In other words, the phases are coherent and alternate as follows: +-+-+-+-+-. When the radiation emitted by such a laser diode array is focused to a spot onto the recording medium, forming an image of the ten small stripes themselves, called the "near field" image, there may be unacceptably large variations in intensity across the spot. These variations can be troublesome where they exceed the threshold to burn-off ratio, causing nonuniform printing across the spot.
Alternately, when the radiation emitted by such a laser diode array is focused to a spot onto the recording medium by forming an image of the "far field" diffraction pattern of the source array, the alternating phases of the individual stripe elements will typically result in a splitting of the spot into two parts, with a dark center line. This far-field image also has an undesirable variation of intensity.
Attempts have been made in the prior art to solve the alternating phase problem and, thereby, the non-uniform spot illumination problem in the far-field image by placing a phase grating directly on the diode array in order to compensate the phases of the diode outputs. However, such a solution has been found to be undesirable in practice because such phase gratings are difficult to fabricate and tend to be damaged when exposed to the high intensity radiation used for laser printing.
As a result, there is a need in the art for an apparatus which will provide a substantially smooth, uniform image of a multiple-element laser diode array which has substantially alternating phase output from the individual lasers. Further, there is a need for such an apparatus which will provide the substantially smooth, uniform image even when the individual elements of the array are not equally bright and are not perfectly stable in phase.